Attention has been drawn to a p-n-p-n- junction device which is applied to the following optical switch, an optical amplifier, an optical memory, an optical wavelength conversion means, etc. in the field of optical computing, an optical interconnection, an optic-electric integrated circuit, etc. One such p-n-p-n junction devices has been described on pages 596 to 600 of "J. Appl. Phys. 59(2), 15 Jan. 1986" as entitled "A new double heterostructure optoelectronic switching device using molecular beam epitaxy" by G. W. Taylor et. al. The p-n-p-n junction device comprises a buffer layer of n-GaAs having an impurity density of 5.times.10.sup.17 cm.sup.-3 and a thickness of 0.5 .mu.m, a cathode layer of n-Al.sub.0.4 Ga.sub.0.6 As having an impurity density of 5.times.10.sup.17 cm.sup.-3 and a thickness of 1 .mu.m, a p-gate layer of p-GaAs having an impurity density of 1.times.10.sup.19 cm.sup.-3 and a thickness of 50 .ANG., an n-gate layer of n-GaAs having an impurity density of 1.times.10.sup.17 cm.sup.-3 and a thickness of 1 .mu.m, an anode layer of p-Al.sub.0.4 Ga.sub.0.6 As having an impurity density of 5.times.10.sup.18 cm.sup.-3 and a thickness of 1 .mu.m, and a cap layer of p-GaAs having an impurity density of 1.times.10.sup.19 cm.sup.-3 and a thickness of 0.1 .mu.m which are grown sequentially on a substrate of n-GaAs by using molecular beam epitaxy. The p-n-p-n junction device further comprises a ring shaped p-electrode of AuZn alloy and an n-electrode of AuGeNi alloy which are provided on the top surface of the cap layer and the back surface of the substrate respectively.
The device is considered to be a thyristor having a p-n-p-n junction so that a switching operation can be performed therein. In operation, a switching voltage under which the device is turned on is shifted in the direction of a lower voltage in a case where an input light is supplied thereto. Although substantially no electric current is passed through the device in the OFF state, electric current is passed therethrough in the ON state. The electric current which is passed in the ON state is varied dependent on a biased value determined by a load resistance which is connected in series to a power source. On this occasion, light emission is induced in the presence of a light emitting layer of the device. For the reason described above, the ON state is obtained in the device in accordance with the switching-on thereof, when light is supplied thereto in a case where a voltage slightly lower than a switching voltage is applied to the device. Therefore, the device can be used for an optical switch, an optical logic device, etc. because light emission is obtained therein under the ON state.
According to the conventional p-n-p-n junction device, however, there is a disadvantage that in an optical part like an optical isolator, a beam splitter, etc. must be provided in an optical system thereby making the cost of the optical system higher. This is true because the input and output lights are transmitted on a common light path since the input light is supplied to a light input/output window, and the output light is radiated from the light input/output window or from a light output window which is provided on an opposite surface of the device. For the same reason, the alignment of light axes is difficult to performed with input and output lights, especially in the case where a plurality of input or output lights are utilized in the device.
Further, there is another disadvantage in that wavelengths are deviated between the input and output lights such that application of the device is difficult, especially, to an optical amplifier. This is one reason why the receiving and emitting of the lights are performed in the n-gate layer thereof whereby light is received in a wavelength shorter than a forbidden band wavelength of the n-gate layer, while light is emitted in a wavelength approximately equal to the forbidden band wavelength. Thus a wavelength is shifted in the direction of a longer wavelength in the case where the device is used for an optical amplifier or an optical wavelength conversion means.